For many decades, self-propelled combine harvesters have been used by farmers to harvest a wide range of crops including cereals, maize and oil-seed rape. Typically, a combine harvester cuts the crop material, threshes the grain (or seed) therefrom, separates the grain from the straw, and cleans the grain before storing in an on-board tank. Straw and crop residue is ejected from the rear of the machine.
WO-2012/095239 discloses a combine harvester having threshing and separating apparatus arranged to convey a cut crop stream in a generally rearward direction. A threshing cylinder rotating on a transverse axis conveys the crop stream tangentially underneath. Grain dislodged from the crop stream falls through a concave grate onto an underlying oscillating pan, hereinafter termed a stratification pan.
The remaining crop stream is conveyed to the separating apparatus which, in this case, comprises a pair of axial separating rotors. Grain and material other than grain (MOG) falls through a grate arrangement disposed under the rotors onto an oscillating return pan which conveys the grain and MOG forwardly to a front edge from where it falls onto the stratification pan.
The grain and MOG collected by the stratification pan is conveyed rearwardly by the oscillating motion, in combination with a ridged pattern on the pan, to a rear edge from where it falls into a cleaning unit having a plurality of sieves and the like.
The return pan of WO-2012/095239 overlaps the stratification pan so that the grain and MOG conveyed by the former falls onto the latter. The stratification pan stratifies the different materials, namely the chaff, straw and grain, as they are conveyed rearwardly before falling into the cleaning unit. Stratification of the grain and chaff allows a reduced cleaning airflow thus minimising grain loss. Furthermore, the reduced entropy of the grain/chaff mix allows the grain to settle more rapidly on the chaffer of the cleaning unit.
Testing of the construction disclosed by WO-2012/095239 reveals significant improvement in the capacity of the cleaning unit. However, efforts continue to improve the stratification of materials on the stratification pan upstream of the cleaning unit.